Clark Terry is one terrific trumpeter, and surprisingly few modern listeners seem to know it. Throughout the 50s he starred with Duke Ellington’s bands, and even among that crew of unique instrumental voices (which Ellington managed to blend into his own unmistakable sound), Terry stood out. But perhaps, as with Dizzy Gillespie, his clownish wit and bumptious good humor got in the way, preventing a fuller appreciation of his speedy technical mastery of trumpet (both open and, especially, muted) and flugelhorn. Terry’s comedic-talents came to the fore in his “Mumbles” routine of not-quite-verbal scat-singing, which he offered a national TV audience as part of the Tonight Show orchestra in the 60s, but his horn playing stems from the some sunny disposition: even his blue notes sound like harbingers of good times to come. Terry has a long history of advising younger players–back when he still lived in Saint Louis he befriended the teenage Miles Davis (who counted the virtuosic Terry among his first musical idols)–and he adds to it next week by performing with the Northern Illinois University big band. Band director Ron Modell has made a habit of inviting top-notch instrumentalists to front his always strong student band, and the experience not only raises the collegians’ level of play but usually seems to please the pros as well. Wednesday through next Saturday, December 19, Joe Segal’s Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 4274846. (Terry returns to the Showcase nine days later for a New Year’s gig, December 29 through January 3, in the company of peers–saxist Red Holloway, Chicagoans Larry Gray on bass and Willie Pickens on piano, and Terry’s fellow former Ellingtonian, Louis Bellson, one of the world’s great drummers.)